1. Field of the Invention
The invention relates to a magnetic-tape-cassette apparatus comprising a servo device for positioning at least one part of the apparatus. The servo device comprises a servo wheel having circumferential teeth around a major portion, and at least one untoothed portion which, in a rest position of the servo wheel, faces a motor-driven gear wheel. The servo wheel carries at least one latching projection as well as a cam which, in the rest position of the servo wheel, is subjected to a starting torque produced by spring force. The servo device further includes a relay-armature member which is pivotable between a latching position and an inoperative position, and which carries a soft iron element which in the rest position of the servo wheel is attracted by an impulse relay to maintain the relayarmature member in the latching position. A stop projection on the relay-armature member cooperates with the latching projection on the servo wheel when the member is in the latching position, to keep the servo wheel latched in the rest position.
After a brief electrical energization of the impulse relay, the relay-armature member is pivoted under spring force from the latching position to the inoperative position, in which the stop projection is clear of the latching projection. Under the influence of the starting torque exerted on the eccentric cam, the servo wheel is then rotated to bring the teeth of the servo wheel into mesh with the gear wheel so that the servo wheel is be driven by the motor to position the desired part of the apparatus. After this the latching projection cooperates with a reset projection on the relay-armature member to return the relay-armature member against the spring force from the inoperative position to the latching position; and the latching projection subsequently abuts the stop projection so that the latter latches the servo wheel in the rest position.
2. Description of the Prior Art
Such a magnetic-tape-cassette apparatus is known from a document published by KISHO-Electronics Co., Ltd. (Japan). The magnetic-tape-cassette apparatus proposed therein comprises a relay-armature member which keeps the servo wheel latched in the rest position under the influence of an impulse relay. Such an impulse relay comprises a permanent magnet which in the latching position of the relay-armature member firmly attracts this member by means of the soft iron element. Around the magnet a coil is arranged to form an electric current source by means of a switch. The construction and arrangement of the electromagnet relative to the permanent magnet is such that the permanent magnet is rendered magnetically neutral while the current source is switched on. As a result, the relayarmature member is no longer attracted and a spring can cause the relay-armature member to be pivoted to the inoperative position, so that the servo wheel is released and can be rotated.
The advantage of such an impulse relay is that the current source need only be switched on for a short time, which is adequate to allow the impulse relay to be rendered magnetically neutral for a short time and the relay-armature member to be pivoted. This brief switching-on has the advantage that the current source of the magnetic-tape-cassette apparatus is loaded only briefly and the impulse relay consumes only a small current. This is not only an advantage for battery-powered apparatus but it is also advantageous for modern microprocessorcontrolled equipment, in which such a brief energization is simple to achieve. Moreover, such an impulse relay can be of compact construction and has only a very small heat dissipation in comparison with a conventional electromagnet.
In the magnetic-tape-cassette apparatus described in this Japanese document the various parts of the relay-armature member have to be positioned accurately relative to the cooperating parts of the servo wheel and the impulse relay in order to ensure that in the latching position, when attracted by the impulse relay, the servo wheel is latched effectively. However, this accurate positioning is difficult to achieve due to the inevitable tolerances of such a servo device, which in practice means that the servo device of the known apparatus is comparatively expensive owing to the accuracy required during manufacture and assembly.